Published by Eric Bogatin on 07 Aug 2012 at 01:24 pm
The second instructor at the IEEE Global EMC and SI University was Prof Tzong-Lin Wu, of the Dept. of EE, National Taiwan Univ. He flew in from Taiwan last night on a 22 hour flight to be here in Pittsburg, and showed no signs of jet lag.
While most of his presentation was setting the foundation of transmission line theory, he also spent time talking about return paths. This is the growing theme for this Global University. In particular, he offered a pop quiz to the 50 attendees.
Of course, the two extremes are obvious. Case 1 will be best and case 2 will be the worse. After all, it’s important for the signal to never cross a split in the return path. But what about case 3 and 4? Which is worse, and by how much worse?
Our intuition may suggest that case 4 will be much worse than case 3. If the return path sees a continuous return, there should be little radiated emissions. So, shouldn’t case 3 be much better than case 4?
Each of these four cases were built in simple circuit boards and the radiated emissions measured by students. Their results are shown here.
Case 2, the signal crossing the gap, shows the most radiated emissions.
Case 4, with the signal hopping over small squares of holes in the return plane, creates the next most emissions. This was sort of expected. What is really interesting is that the case 1 and 3, with a continuous return plane, are identical. This says, even though there were gaps in the return path, but outside the width of the signal line, the gaps played no role in radiated emissions. It only takes a little web of continuous return path under the signal line to control the radiated emissions.
This was a great example of the dance between theory, measurement and simulation to illustrate the importance of return paths.